Indirect printing methods generally include a two-step printing process including applying ink imagewise onto an intermediate receiving member, such as a drum or a belt, using an inkjet printhead, and then transferring a transient image to a substrate. After the ink is applied imagewise onto the intermediate receiving member, the ink wets or spreads on the intermediate receiving member to form a transient image. The transient image undergoes a change in properties, such as partial or complete drying, thermal or photo-curing or gelation, and is then transferred to the substrate.
Inks for use in an indirect printing method are designed and optimized to be compatible with the different subsystems, i.e., jetting and transferring. Specifically, an ink used in indirect printing must have properties, such as surface tension, viscosity, and particle size, suitable for use in a piezoelectric inkjet printhead. The ink must also be able to wet the intermediate receiving member to form the transient image and to undergo a stimulus induced property change to release from the intermediate receiving member in the transfer step.
Particularly, inks suitable for use in indirect printing must meet specific sub-system requirements that are unique to the inkjet/transfix printing architecture. Two important properties include wetting and release properties. Currently, no ink exists that enables both wetting and transfer that enables high quality printing at high speeds. Generally, inks that display good wettability do not transfer well onto a substrate and conversely, inks that efficiently transfer to the substrate do not adequately wet the intermediate receiving member.
Conventional aqueous latex inks contain water, one or more humectants, a colorant, and a polymer latex. A polymer latex is an aqueous dispersion of polymer particles having a size below 500 nm. When typical emulsion polymerization is used to produce a polymer latex, the particle size is largely determined by the surfactant concentration, the monomer choice, and the temperature. Latex properties such as stability, film-forming ability, covering capacity, and opacity are controlled by particle size and the amount of surfactant present. Generally, lower particle size improves the above properties, particularly film formation. However, one problem with conventional latexes is that to get a particle size below 100 nm requires large amounts of surfactants, which detrimentally affect latex properties.